Definition

The steradian (symbolized sr) is the Standard International (SI) unit of solid angular measure. There are 4 pi, or approximately 12.5664, steradians in a complete sphere.

A steradian is defined as conical in shape, as shown in the illustration. Point P represents the center of the sphere. The solid (conical) angle q, representing one steradian, is such that the area A of the subtended portion of the sphere is equal to r2, where r is the radius of the sphere.

A general sense of the steradian can be envisioned by considering a sphere whose radius is one meter (r = 1m). Imagine a cone with its apex P at the center of the sphere, and that intersects the surface in a circle (shown as a red ellipse, the upper half of which is dashed). Suppose the flare angle q of the cone is such that the area A of the spherical segment within the circle is equal to one meter squared (A = 1 m2). Then the flare angle of the cone is equal to 1 steradian (q = 1 sr). The total surface area of the sphere is, in this case, 12.5664 square meters (4 pi times the square of the radius).

Based on the foregoing example, the geometry of which is independent of scale, it can be said that a solid angle of 1 sr encompasses about 1/12.5664, or 7.9577 percent, of the space surrounding a point.

The number of steradians in a given solid angle can be determined by dividing the area on the surface of a sphere lying within the intersection of that solid angle with the surface of the sphere (when the focus of the solid angle is located at the center of the sphere) by the square of the radius of the sphere.

Also see radian and Standard International (SI) System of Units.

Contributor(s): Tom Higgins
This was last updated in September 2005
Posted by: Margaret Rouse

Register now to receive SearchCIO-MidMarket.com-related news, tips and more, delivered to your inbox.
By submitting you agree to receive email from TechTarget and its partners. If you reside outside of the United States, you consent to having your personal data transferred to and processed in the United States. Privacy

More News and Tutorials

• Remote backup can ease network disaster recovery

Backing up to local disk or tape can protect against the accidental destruction of a file but will do nothing to protect you when your facility suffers fire or flooding. Remote backup provides a better solution. This tip looks at two possibilities for remote backup: Choosing an outsourced remote backup service, or using software to do your own backups to an offsite facility.

• VoIP now part of phishing attacks

Learn how attackers are using the widespread deployment of low-cost VoIP to leverage phishing attacks.

• VoIP privacy on the WAN

Is the government listening to your VoIP traffic? Can other companies spy on your WAN? It may sound like a paranoid conspiracy theory, but it's not that far out, technologically speaking. In this tip, Tom Lancaster looks at just how exposed your WAN traffic may be.